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than anteriorly. Although present to a slight extent at first anteriorly, it never develops greatly. This means that the zone of special activity is at a later stage more intense posteriorly than anteriorly. So apparently it is at an earlier stage, and causes the greater bulkiness of the posterior end of the blastodermic vesicle of the seventh day, and causes the apparent throwing forward of the embryonic disc (v. “175 hr.," fig. 42).

Table of Measurements of Thickness of Albumen

Layer at Different Parts of the Vesicle in Mil. limetres.

Five Days.

Six Days Four Hours. Embryonic area

0060

0058 Placental zone, A.

0024

·0022 Placental zone, P.

0030

.0024 Lower pole

0036

0034

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To sum up the above section :

(1) There is no growth round of the inner mass cells over the surface of the outer layer cells in the sense of a migration. It is only an apparent growth round produced by the more rapid growth of a zone of the wall of the vesicle immediately surrounding the embryonic disc, in which zone the marginal cells of the inner mass lie. (2) The

presence of such a zone accounts in a great measure for the shape assumed by the blastodermic vesicle during the fifth, sixth, and seventh days.

(3) A zone of such a character undoubtedly exists during the eighth, ninth, and following days, giving rise to the ectoplacenta.

(4) Such a zone of activity accounts for the varying thick. ness of certain parts of the albumen layer.

To return for a time to the question of the growth round of the inner mass cells. It must be remembered that the cells never completely surround the cavity of the blastodermic vesicle. The lower pole is always, in the rabbit, one-layered only as long as it exists.

There is only one other alternative that will account for the apparent growth round of the inner mass on the inner wall of the blastodermic vesicle, as far as I can see, and that is actual active migration of the cells in question. Of course it is difficult to bring evidence to show that they have not migrated, since it is not possible, I fear, to follow the process in any one and the same specimen. At the same time I cannot find any evidence to prove that they have actually migrated.

If a cell does migrate, like an Ameba for instance, one would expect to find evidence of protoplasmic protuberances or pseudopodia. Of this I can find no trace. The majority of the cells seem at first (figs. 38, 39) to be quite isolated from each other, and to be approximately spherical, whether examined in the perfectly fresh condition or after treatment with various reagents. They are, it is true, slightly flattened on the side by which they adhere to the vesicle wall (fig. 28, HY. I.).

Certain of the cells here and there are connected by threads of protoplasm, but this, I think, is not a sign of pseudopodic activity, but merely indicates the final stage in division between the two cells. I have no doubt that these cells divide rapidly after a time, though I do not think much activity of division takes place during the first few hours after the apparent migration begins.

If one of these inner rounded cells is undergoing the process of division, then, as the wall on which it rests expands, the two dividing halves of the inner cell will be pulled apart, and a strand of protoplasm connecting the two cells may remain for some time.

Of course it is just possible, I suppose, that these rounded inner cells might migrate by means of a rolling motion consequent on “streams” of protoplasm within them, as do some protozoa. But is such a phenomenon known anywhere in the metazoan body? I cannot think we are justified in assuming this without evidence, for when examined in the fresh condition no such protoplasmic activity can I notice.

When the cells are so isolated as I believe them to be during VOL. 37, PART 2.— NEW SER.

K

the 98th to 108th hours or thereabouts in the rabbit, I cannot conceive that the migration can be accomplished otherwise than by an actual migration, for which there is no evidence, or else by a process such as I have attempted to describe above. This I believe to exist. If the inner layer was not composed of isolated cells, but was a compact membrane, then it might creep round by means of its own interstitial growth, although I do not think it would in that case be a thin smooth membrane.

Such is the account of the growth round of the hypoblast in the mole. Heape makes no mention of any isolated cells at the edges; he says of the hypoblast, “it extends laterally by virtue of the multiplication of its cells, which at the same time become much flattened.” It seems to me to be much more likely that the layer would be flattened if they were drawn out along with the epiblast cells; but I have no evidence at present whether the same cause can be attributed to the spread of the hypoblast in the mole as I suggest for the rabbit.

In fig. 42 the dotted lines radiating from the centre of the smallest blastocyst indicate the amount of growth of the several segments up to the one hundred and seventy-fifth hour.

Until the one hundredth hour I imagine the growth of all parts of the wall of the blastocyst to be equal.

From that moment there is a greater activity in an area around the embryonic disc, which causes the inner layer to be apparently carried further round the anterior of the blastocyst.

This zone of activity by the one hundred and fortieth hour has become more marked still, and now shows itself to be more intense posteriorly than anteriorly, which latter character is plainer still at the one hundred and seventy-fifth hour.

Eventually the activity culminates, upon the addition of resistance afforded by the walls of the uterus, in the production of the ectoplacental area, or part of it.

To this I have referred in another paper, and also to the hypothesis that this zone of activity, in the absence of a tough albuminous coat, results in certain other rodents in the production at once of a heaping up of cells—the träger.

In the embryonic disc region there is very much less activity

exhibited. The outer layer in that region is very much attenuated, and shows very little sign of activity. The inner layer consists all this time of rounded or ellipsoidal cells, and, like the outer layer of epiblast of that region, shows very little if any sign of activity.

So little activity of division does there seem to be in the inner layer of epiblast, that there is a distinct tendency for the several cells to become slightly separated as in fig. 30, which gives rise to the very irregular and speckled appearance of the embryonic disc of the one hundred and twentieth hour.

Very probably this is caused by the slight stretching of this region. It is more noticeable at the edges than towards the centre.

Whether there is any palingenetic meaning in this doublelayered condition of the epiblast I have discussed in another paper. For the present, I think the right view to take of the condition is that derived from the study of the actual way in which the separation has originated, and to regard it as a consequence of ontogenetic circumstances only.

The Outer Layer of Epiblast. This is by far the most active of the embryonic layers of the fifth day. It is in an active condition of growth during the whole of the day, and thereby allows of the expansion of the vesicle. The character of the cells seems to be just as it was during the latter part of the fifth day.

The Inner Layer of Epiblast.—This layer seems to be a region of rest throughout the whole of the sixth day. There is very little sign of cell multiplication. The cells are more or less circular in outline when viewed from above, and oval when seen laterally. They are rather scattered, and thus give rise to the speckled appearance noted above. They seem to be clearly separated from the outer epiblast above them and from the hypoblast below them, and since they are either quite separated from each other, or connected only by fine strands of protoplasm at certain minute spots, they are simply pulled apart by the expansion of the blastodermic vesicle, and are not individually stretched and flattened,

as are the cells of the outer epiblast and hypoblast, where they are more intimately connected one with another. In the latter case, that of the hypoblast, cells with a tendency to the features characteristic of both layers of epiblast are to be found.

CHAPTER IV. CHANGES THAT OCCUR DURING THE Sixth Day (120th to

141th Hours). If we may call any period of the development of an animal unimportant, it is to the period between the 120th and 144th hours of the development of the rabbit that this epithet might be applied.

During this day the blastodermic vesicle increases very greatly in size, and assumes very markedly the shape described in the last chapter as being characteristic of the later stages of the development of the vesicle, prior to its attachment to the walls of the uterus.

The blastodermic vesicle is no longer a sphere. It will be found by measurement to have longer and shorter equatorial axes, and a polar axis which is of less length than the shortest equatorial axis.

The following measurements are taken from specimens of the earlier part of the sixth day :

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(1) Hypoblast of the Embryonic Disc.—This is now a continuous layer, sufficiently so as to show lines of demarcation between the cells when treated with silver nitrate. This continuous membrane extends a short distance beyond the periphery of the inner epiblast layer. The cells composing this membrane are completely flattened.

(2) Hypoblast beyond the Embryonic Disc.—The

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